Assessor-based disease activity measures such as the Disease Activity Score in 28 joints (DAS28), although widely used in rheumatoid arthritis (RA), have high interobserver variability. We developed and validated a patient-based disease activity score (PDAS) as an alternative assessment.
Patients' assessments of swollen or tender joints, visual analog scales for pain and general health, the Health Assessment Questionnaire, and erythrocyte sedimentation rate (ESR) were used to develop the PDAS. In a developmental cohort (204 patients), regression analyses determined the best fit with the DAS28. A validation cohort (322 patients) subsequently evaluated criterion and construct validity against a range of outcome measures, including the Nottingham Health Profile (NHP) and Short Form 36 (SF-36). Sensitivity to change was assessed in 56 patients after 6 months of treatment with disease-modifying antirheumatic drugs or biologics.
In the developmental cohort, the PDAS with ESR (PDAS1) and without ESR (PDAS2) achieved excellent fit with the DAS28 (r = 0.88 and 0.74, respectively). In the validation cohort, the PDAS showed high criterion validity by correlation with the DAS28 (PDAS1: r = 0.89, PDAS2: r = 0.76). Construct validity was demonstrated by high correlations with a range of disease activity measures (r ≥ 0.45), whereas low correlations (r < 0.45) with mental and social components of the SF-36 and NHP indicated divergent validity. The PDAS and DAS28 had similar sensitivity to change, determined using effect sizes (DAS28 = 1.03, PDAS1 = 1.02, PDAS2 = 0.77) or standardized response means (DAS28 = 0.79, PDAS1 = 0.77, PDAS2 = 0.73).
The PDAS1 and PDAS2 are valid and sensitive tools to assess disease activity in RA. They appear suitable for clinical decision making, epidemiologic research, and clinical trials.